Diseased Justin Brouwer Boxwood
This may be volutella stem blight, in which case you can cut off the branch as far down as possible, and hope that it is not also infecting other branches. However, we also see foliage starting to yellow, which could indicate that more of the plant is infected.
Here's info on volutella (and other causes of boxwood dieback): https://extension.umd.edu/hgic/topics/boxwood-preventing-and-managing-common-pests-and-diseases Scroll down for elaboration on each cause.
Even if the entire plant is not going to die, once this dead part is pruned out you'll have a boxwood doughnut. It would take many years to fill in, as this dwarf boxwood is very slow-growing. We'd recommend replacing it.